The present invention relates to low temperature heat sealable, biaxially oriented polypropylene composite films having improved lubricity, anti-blocking properties and scratch resistance.
Biaxially oriented crystalline polypropylene films are known to possess a number of advantageous characteristic properties including excellent optical properties such as transparency and brilliance, satisfactory mechanical properties such as tensile strength and Young's modulus, exceptionally low vapor-permeability, and substantial non-toxic and odorless properties, and accordingly they are widely used as packaging material, particularly for foods. However, the biaxially oriented crystalline polypropylene films have defects in that as a single substance they exhibit rather poor heat sealability and even if they are heated to a temperature which enables their heat-sealing, they cannot usually provide a satisfactory level of heat-sealing strength. In addition, when they are heat-sealed, wrinkles due to heat shrinkage usually appear on the surface. Such wrinkles spoil the appearance of an article which is wrapped with such a film and decrease the value of the same as an article of commerce.
To obtain improved films free from such defects, a variety of trials have been made heretofore. Representative examples of such trials include the application of a readily heat-sealable resin to one or both surfaces of said biaxially oriented crystalline polypropylene film by such means as: (1) by a partial coating method; (2) from a solution or an emulsion of the coating resin; (3) by a dry laminating method; (4) by a melt extrusion laminating method, or the like. However, none of these trials has provided fully satisfactory results, because these methods also are not free from various problems: with the application of a coating layer, there is a problem of a substantial cost increase due to the use of an additional process step; with the use of organic solvents there are such problems as how to recover the used solvent, how to avoid pollution of the working atmosphere; the dietary hygiene problem and others. Accordingly, various other processes for obtaining biaxially oriented polypropylene films with satisfactory characteristic properties without using such organic solvents as mentioned above and without special treatment to the surface of the films have been proposed and tried. For example, there are many known processes for the production of bi-axially oriented polypropylene composite films comprising the steps of laminating a specified resin given hereinbelow on at least one surface of the substrate film before the resulting laminated film is either uni-axially or bi-axially oriented. The resins to be used for such purpose include, for example, ethylene-propylene copolymer as disclosed in Japanese Patent Publications Nos. 39275/72 and 14343/74; middle or low density polyethylene and ethylene-vinyl acetate copolymer as disclosed in Japanese Patent Publication No. 11353/66; ionomer resin as disclosed in Japanese Patent Publication No. 40793/71; syndiotactic 1,2-cis-polybutadiene as disclosed in Japanese Patent Publication No. 50068/77; ethylene-(1-butene)copolymer as disclosed in Japanese Patent Public Disclosures Nos. 109577/77, 104585/77, and 12971/77, etc.
From the viewpoint of heat sealability of these resins, ethylene-vinyl acetate copolymer, ionomer resin, syndiotactic 1,2-cis-polybutadiene, and ethylene-(1-butene)copolymer can provide a practically acceptable sealing strength at a temperature of 100.degree. C. or lower, while middle or low density polyethylene provides a practically acceptable level of heat sealing strength at a temperature of 100.degree.-130.degree. C. and ethylenepropylene copolymer provides such heat sealing strength at a temperature of 130.degree.-150.degree. C.
From the viewpoint of packaging machine suitability, a composite film should preferably provide an acceptable heat sealing strength at a temperature of 100.degree. C. or lower. However, resins such as ethylene-vinyl acetate copolymer, ionomer resin and syndiotactic 1,2-cis-polybutadiene have poor heat resisting properties and readily deteriorate owing to cross-linking, deacetylation, etc. In addition, they have poor compatibility with polypropylene resin. Accordingly, if they are added to polypropylene resin, the physical properties such as appearance and transparency of the polypropylene film prepared therefrom will be impaired substantially, and as a result, the value of merchandise made from or packaged with the final film product will be decreased significantly. Accordingly, if these resins are used as laminating resins, trimmings and/or defective film products obtained from these laminating resins cannot be recycled. Namely, they cannot be added to a raw material polypropylene from which a substrate film is prepared, because they have the defects mentioned above, and since the recycling is impossible for these reasons, the cost of production increases.
Resins which are heat-sealable at a temperature of 100.degree. C. or lower are generally tacky and have a low melting point and low crystallinity. Therefore, they have rather poor lubricity and poor anti-blocking properties, and do not always have satisfactory scratch resistance. So, it is difficult to use these resins as a single substance in the practical film production process. By incorporating various known additives such as a lubricant (such as a higher fatty acid amide), an anti-blocking agent (such as silica) and a resin selected from polyolefinic and other resins, such defects as mentioned above have been remedied to a certain level and, accordingly, for some specific uses, satisfactory films have been obtained. In general, however, films having a satisfactory balance of various characteristic properties has not yet been obtained.
In contrast, resins such as ethylene-(1-butene)copolymer, middle and low density polyethylenes and ethylene-propylene copolymer have rather good compatibility with polypropylene as well as rather good heat stability. Therefore, when these resins are used, recycling is possible within a certain limit, and for this reason their influence on the cost of production is not so serious. However, since ethylene-(1-butene)copolymer is also a tacky resin having a low melting point and low crystallinity, it is not free from the same or similar defects as mentioned previously with respect to the aforementioned low melting resins. In addition, since middle and low density polyethylenes and ethylene-propylene copolymer have a higher heat sealing temperature and a narrower heat sealable temperature range, they do not fit well into the usual operational sequence when used in a high-speed automatic packaging process.
As is obvious from the above explanations, the various trials for improving the heat-sealability of a polypropylene film by laminating a readily heat sealable resin layer on one or both surfaces of the polypropylene substrate film have not yet been fully successful.
In view of these circumstances, we have studied from every point of view the problem as to what kind of a readily heat sealable resin should be selected and how it should be laminated on the polypropylene substrate film. We have changed the types of laminating resins and the thicknesses of the lamination layers. We have changed the kinds and amounts of the additives to be incorporated in the laminating resins such as lubricants, anti-blocking agents, antistatic agents and the like. And yet, we have found that it is very difficult to improve the heat sealability of an oriented crystalline polypropylene film without impairing its many advantageous intrinsic characteristic properties and obtain a polypropylene composite film which has well-balanced excellence in all properties--lubricity, anti-blocking property, anti-scratch property and low temperature heat sealability.
As a result of our very earnest study, however, we have finally accomplished the present invention relating to a novel process for the production of low temperature heat sealable polypropylene composite films having improved lubricity, improved anti-blocking properties and improved scratch resistance in addition to excellent suitability for a high speed automatic packaging operation. Thus, the process of our invention ensures a substantial reduction in cost of production without impairing superior characteristic properties such as superior transparency, superior brilliance, superior mechanical properties and the like, which are intrinsic to bi-axially oriented polypropylene films.